MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 273 



an oval shape in birds). The substance of the occipital bone is thick and richly 

 cellular in the Balseniceps. 



In the Chick, the occipital sclerotome is developed from five centres ; one basal, a pair 

 of laterals, and a pair of upper or ' epiotic ' pieces. The basi- occipital of the Chick may be 

 seen, on the eleventh day of incubation, as a short rod of bone in the upper stratum of the 

 cartilage of the primordial basis cranii ; it encloses the tapering anterior extremity of the 

 evanescent chorda dorsalis, but has not yet penetrated the thick cartilage beneath it, and 

 is only just entering the substance of the, as yet, cartilaginous hemispherical condyle. , 



The basal centre lies on the same plane as the centre of ossification, which already 

 occupies the posterior third of the basi-sphenoidal rostrum. The lateral or ex-occipitals 

 may be seen at the same period as thin scales of bone of a somewhat crescentic shape, 

 bounding the sides of the foramen magnum (hence their crescentic margin), sending 

 upwards a process to join the epiotics, and another forwards and outwards to form the 

 'par-occipital' ala, and to ossify that part of the occipital sclerotome which in the 

 Chelonia exists as a distinct ' mastoid.' 



On the eleventh day of incubation most of the upper occipital region is still cartilagi- 

 nous ; but on each side of the mesial line, a little above the foramen magnum, a pair of 

 small oval osseous centres already exist ; they are about a line apart. 



On the fourteenth day they are still nearly the same distance from each other ; but 

 they are creeping up to the upper margin of the cartilaginous cranial wall, downwards 

 nearly to the foramen magnum, and laterally they have begun to wall-in the superior 

 semicircular canals. 



On the sixteenth day these pieces (called ' epiotic bones ' by Prof. Huxley, Croon. 

 Lect. p. 13) have reached each other, as well as the upper margin of the occipital car- 

 tilage and the superior boundary of the foramen magnum. 



On the nineteenth day only the upper and lower fourths of the line of union of these 

 ' epiotics ' are visible, and the external margin has reached the external as well as the 

 superior semicircular canal. 



In young Pigeons, a day after hatching, these ossifications have not commenced ; on 

 the eighth day these rapidly-growing birds have a large single ' supra-occipital ' deeply 

 notched in the middle, inferiorly, in the place of the oval membranous deficiency, or 

 fontanelle, in the primordial occipital wall. In adult Pigeons, and also in the Dodo and 

 Didunculus, traces of this structure still remain (see Strickland and Melville), — all the 

 ColumbiuBe hitherto examined having a mesial ' supra-occipital ' foramen. These birds 

 must be examined on about the third day for the separate epiotic pieces'. 



The broad smooth supra-occipital region of birds does not require a distinct inter- 

 parietal, or central supra-occipital element ; but in the Chelonia, and probably in most 



' As early as the third day after incubation, we find (from dissections made since the above was written) that 

 in these typical birds there is only one osseous centre in the supra-occipital cartilage : it is shaped like a horse- 

 shoe and is very rapid in growth. 



